Our overall goal of this project is to establish methods for blocking protein-protein interactions with small organic molecules. Such inhibitors could selectively prevent the assembly of particular complexes and thus disrupt downstream processes. Often such complex formations are mediated by modular protein binding modules such as the WW, PDZ, and SH3 motifs. Recently, several structural studies have revealed a new class of protein interaction. This binding mode involves the fitting of one face of a small helical segment of one protein into a well-defined cleft on the other. One example is the binding of glucocorticoid receptor-interacting protein 1 (GRIP1) to the human thyroid hormone receptor beta 1 (hTR=DF1) ligand (T3) complex. Due to the rather small size of the overall binding site for the helix and the absence of backbone hydrogen bonding between the two partners, this type of interaction represents a promising test case for small molecule inhibitors of protein-protein binding. The long term research goal of the project is to develop a cell permeable inhibitor for GRIP1/hTRb1/T3 binding, specifically (1) to develop a robust biochemical assay suitable for high throughput screening for competitive inhibitors of the binding of GRIP1 to hTR=DF1/T-3 complex; (2) to synthesize a conformationally constrained a-helical peptide that spans the putative binding portion of GRIP1 and test this template as an inhibitor; (3) to explore the minimal structural constraints for the binding of the GRIP1 helix to the hTR=DF1/T-3 complex; and, (4) to begin to define the structure-activity relationship for the GRIP1 helix using peptide models. The Computer Graphics Laboratory resources are being utilized to analyze the interaction between ligand and receptor and to design potential inhibitors based upon this analysis.